The present invention relates generally to an improved method of molding a thermally conductive article from composite material. More specifically, the present invention relates to a method of molding a thermally conductive article that is easily moldable while yielding thermal conductivities many times greater than prior art molding methods even employing known materials.
In the heat sink industry, it has been well known to employ metallic materials for thermal conductivity applications, such as heat dissipation for cooling semiconductor device packages. For these applications, such as heat sinks, the metallic material typically is tooled or machined from bulk metals into the desired configuration. However, such metallic conductive articles are typically very heavy, costly to machine and are susceptible to corrosion. Further, the geometries of machined metallic heat dissipating articles are very limited to the inherent limitations associated with the machining or tooling process. As a result, the requirement of use of metallic materials which are machined into the desired form, place severe limitations on heat sink design particular when it is known that certain geometries, simply by virtue of their design, would realize better efficiency but are not attainable due to the limitations in machining metallic articles.
It is widely known in the prior art that improving the overall geometry of a heat dissipating article, can greatly enhance the overall performance of the article even if the material is the same. Therefore, the need for improved heat sink geometries and lower cost necessitated an alternative to the machining of bulk metallic materials. To meet this need, attempts have been made in the prior art to provide molded compositions that include conductive filler material therein to provide the necessary thermal conductivity. The ability to mold a conductive composite enabled the design of more complex part geometries to realize improved performance of the part.
The attempts in the prior art included the employment of a polymer base matrix loaded with a granular material, such as boron nitride grains. Also, attempts have been made to provide a polymer base matrix loaded with flake-like filler material. These attempts are, indeed, moldable into complex geometries but still do not approach the desired performance levels found in metallic machined parts. It is a known in the art that filler material, particularly high aspect ratio filler material, will align parallel with the flow path of the base matrix within a mold. Therefore, these conductive composite materials must be molded with extreme precision due to concerns of filler alignment during the molding process. This is of concern when the filler material is non-symmetrical or when there is an aspect ratio greater than 1:1 of the thickness to the length of the filler. Even with precision molding and design, inherent problems of fluid turbulence, collisions with the mold due to complex product geometries make it impossible to position the non-symmetrical filler ideally thus causing the composition to perform far less than desirable. This problem is exacerbated when the filler has an aspect ratio greater than 10:1. This is a serious concern because filler is commonly employed that has an aspect ratio up to 40:1.
Moreover, the entire matrix of the composition must be satisfactory because heat transfer is a bulk property rather than a direct path property such as the transfer of electricity. A direct path is needed to conduct electricity. However, heat is transferred in bulk where the entire volume of the body is employed for the transfer. Therefore, even if a highly conductive narrow conduit is provided through a much lower conductive body, the heat transfer would not be as good as a body which is consistently marginally conductive throughout the entire body. Therefore, consistency of the thermal conductivity of the entire matrix of the composite body is essential for overall high thermal conductivity. Moreover, the proper alignment of the loaded filler material, particularly high aspect ratio filler, within the polymer base is of critical importance.
In view of the foregoing, there is a demand for an improved method of molding thermally conductive articles of composite material. In addition, there is a demand for a method of molding that can fully exploit the employment of composite polymer and filler material; namely, the proper alignment and positioning of filler material with the base polymer matrix. There is also a demand for such a method to enable such polymer and high aspect ratio filler compositions to be easily molded into complex product geometries. There is also a demand for such a method of molding to form an article that exhibits thermal conductivity as close as possible to purely metallic conductive materials while being relatively low in cost to manufacture.